Synthetic plastics materials have long been used for the packaging of foods and other materials which need protection from handling and from moisture. However, in recent years, it has become appreciated that, in addition, many foods and other sensitive materials benefit from being protected from atmospheric oxygen and other gases. Barrier films and packaging materials, which are intended to reduce, or inhibit the permeation of gases, vapors, aromas and others, have been extensively described. Common barrier compositions include polyesters, PVDC, acrylic polymers, polyamides and others. PVDC coated films are widely used and exhibit excellent barrier properties against oxygen and moisture vapor even at high relative humidity, thereby improving the gas barrier of a range of base films with otherwise poor gas barrier properties.
Base films can include biaxially stretched films of polypropylene, nylon, or of polyethylene terephthalate (PET) and cellophane among others. Often these substrates may be laminated with other films and employed for wrapping or packing of a variety of foods to protect against gas egress or ingress. Metalized substrates have also been used for packaging materials due to their excellent gas barrier, however drawbacks are cost, and that they have poor flexibility which causes fracture of the barrier metal layer and are mostly utilized as an intermediate layer of a laminated structures.
In the case of PVDC, these packaging materials are disposed of as non-industrial, domestic waste from homes. Unfortunately, when incinerated they give off toxic waste and hazardous gases. Of great concern is the chlorine containing byproducts, which are highly carcinogenic. Therefore, transition to other barrier polymers is highly desired. Polyvinyl alcohol and copolymers such as ethylene vinyl alcohol copolymer have excellent oxygen barrier performance, however this is highly dependent on the ambient relative humidity. The barrier performance at high relative humidity can be improved by incorporation of additives, crosslinking agents such as silanes, multivalent metal cations, and platy fillers but performance above 75% relative humidity is usually diminished.
In order to protect food under typical conditions of ambient temperature and relative humidity, a barrier coating should provide for example <10 cc m2/day oxygen transmission (OTR) at 90% RH and 23° C.; and <10 gm m2/day at 90% RH and 38° C. moisture vapor transmission (MVTR) (other gases often used to modify the atmospheres inside packages such as carbon dioxide are also important). These coatings may be used either as a surface coatings or may be included as part of a multi-layer laminate structure for example for food packaging applications.
Recently, gas barrier coatings have been developed that include platy, or plate-like, fillers to enhance the gas barrier properties. Such platy fillers are typically inorganic laminar materials, also referred to as layered inorganic materials, and generally have a high aspect ratio (i.e. the ratio between the length and thickness of a single ‘sheet’ of material), for example an aspect ratio of greater than about 20 in its exfoliated form, such as between 20 and 10,000. Commonly used inorganic laminar materials have an aspect ratio greater than about 50 for example greater than about 100. Inorganic laminar materials include nanoparticulates, especially nanoparticulate clays. A nanoparticle is a particle having at least one dimension in the nanometer range, i.e. of less than 100 nm. Nanoparticulates used as platy fillers typically have a maximum thickness of less than 100 nm, for example, a maximum thickness of less than 50 nm, such as a maximum thickness of less than 20 nm. Examples of commonly used layered inorganic materials include kaolinite, montmorillonite, atapulgite, illite, bentonite, halloysite, kaolin, mica, vermiculite, diatomaceous and fuller's earth, calcined aluminium silicate, hydrated aluminium silicate, magnesium aluminium silicate, sodium silicate and magnesium silicate. Commercial examples of such inorganic laminar materials are Cloisite Na+ (available from Southern Clay), Bentone N. Dak. (available from Elementis). Microlite 963 and Somasif ME100. It is well known, by those experienced in the formulation of gas barrier coatings, that the inclusion of platy (meaning high aspect ratio) particulates, such as clays, increases the barrier effect, usually by creating a more tortuous path for the gas molecules to penetrate the barrier coating. These particles are usually classified as nanoparticles, which are currently attracting much attention particularly with respect to their toxicology and suitability for food packaging components. Furthermore, while improvements in gas barrier properties are often seen, laminate bond strengths are often reduced.
Polyurethanes have been used in printing, coatings and the like for many years. In the conversion of packaging, there are generally four types of polymer used today.                The first is a low number average molecular weight (5000-25000) polyurethane ‘adduct’ based upon a diisocyanate and long chain diol/triol (e.g. PPG 1000, 1500, 2000; or PTMO 1000, 2000 etc.) which is soluble in organic solvents. This type of polymer does not form a dry film by itself and is used to plasticise harder more brittle resins such as nitrocellulose or cellulose acetate propionate lending better adhesion and flexibility to the films formed.        The second category is elastomeric polyurethanes soluble in organic solvents and are usually based upon a combination of diisocyanates and polyols chosen to impart hard and soft segment domains, which make the polymer a dry film former and typically used in a minimally modified form for lamination ink applications. It is not necessary or indeed practiced to incorporate acid groups into the backbone of solvent soluble polyurethanes, and that modification has been used solely for the purpose of attaining stability in aqueous systems up until now.        The third category is water-borne polyurethane. In order to disperse polyurethanes in water, it is necessary to incorporate a dispersion mechanism since useful polyurethane polymers are insoluble in water. The most common way to achieve this is by incorporating a neutralizable acid group or other anionic hydrophilic group. Commercial water-borne polyurethanes are often neutralized with amines, such as triethylamine, which facilitates neutralization of the carboxyl groups aiding dispersion during chain extensions. Aqueous polyurethane dispersions were found to be slow drying and upon analysis of printed films via gas chromatograph of head space analysis, retained triethylamine was detected above 10 ppb. Although the allowable specific migration limit is expected to be increased to 50 ppb, this limit may well be exceeded at higher film weights and faster press speeds. Whilst it may be possible to substitute this amine with other volatile amines, the drying speed of such coatings is typically slower than solvent soluble polyurethanes which is important for coating speed and production output.        The fourth category is sterically stabilised polyurethanes which are available in water, they are not suited to printing and coating operations.        
The use of polyurethanes for barrier applications has accelerated in the last 15 years with several companies such as PPG, Mitsubishi and Mitsui and others all reported to have developed barrier polyurethanes. However, after evaluating these systems, e.g. WPB 341 from Mitsui, it was found that although performance is promising, they fail to exhibit high gas and moisture vapor barrier under high humidity conditions either as a surface print or in a laminate structure. In order to achieve good gas and/or moisture barrier properties using polyurethane resin coatings, it has either been necessary to include an inorganic filler in the coating composition, to treat the surface of the base film, for example, with a metal oxide layer, or use a metallized film.
US 2005/0084686 A1 (Mitsui) and EP 1 674 529 A1 (Mitsui and Futamura) and U.S. Pat. No. 6,569,533 B1 (Mitsui) are examples of aqueous gas barrier coatings that include dispersed polyurethane resins and layered inorganic materials. The water-borne polyurethane dispersions are prepared from an organic solvent-soluble polyurethane prepolymer, which includes an acid dispersing group such as a carboxylic acid group. The pre-polymer is prepared from reacting an isocyanate, polyol, polyhydroxy acid in an organic solvent, such as methylethyl ketone or acetone. The pre-polymer is then emulsified and reacted with a diamine or other chain-extender to form a water-borne coating composition. The dispersed polyurethane resins of US 2005/0084686 A1 and EP 1 674 529 A1 have a urethane and urea group concentration of 25 to 60 wt % and an acid value of 5 to 100 mgKOH/g. A one-coat system is prepared using the dispersed polyurethane an exfoliated inorganic filler, such as the synthetic mica ME100 or montmorilonite and a polyamine compound or silane coupling agent. The one-coat system is reported as providing a high gas barrier either as a surface coat or within a laminate structure. The polyurethane without any ME100 or montmorilonite is inadequate as a gas or moisture vapor barrier for e.g. food packaging or a suitable replacement for PVDC.
US 2008/0070043 A1 (Toray) report on a gas barrier resin composition comprising (A) a polymer e.g. a polyurethane and (B) an organic compound, e.g. urea, both (A) and (B) containing, a functional group with active hydrogens and/or a polar functional group with a hetero atom. In order to achieve good gas water vapour barrier properties, it is necessary for an inorganic layer, such as a metal oxide layer, e.g. an alumina-evaporated surface, to be applied to the base film underneath the gas barrier resin composition.
Multivalent metal cations have been utilized extensively with polyurethanes either in-situ within the same coating layer as the polyurethane or as a separate coating, either atop or beneath the polyurethane-containing coating, in a multilayer coating system. In these coatings, the function of the metal cation is to either initiate the curing reaction of a polyol or polyamine or combination thereof with a polyisocyanate to deliver specific properties such as hardening or gloss and to accelerate cure times. Multivalent metal cations used as a separate coating has also been reported to improve hardness of polyurethanes. U.S. Pat. No. 7,655,718 B2 (Ecolab Inc.) is an example the use metal cations in order to initiate or enhance the formation of polyurethanes from isocyanates and polyols. That document describes the use of zinc cationic salt initiators for initiating or enhancing the cure of polyurethanes in floor coatings, e.g. by adding zinc ammonium carbonate to an autohardenable polyurethane precursor composition that includes a polyol or polyamine and a polyisocyanate. Alternatively, the zinc carbonate or other initiator for polyurethane hardening may be present in an isocyanate-free undercoat, which is a separate coat to that containing the polyurethane. The polyurethanes of U.S. Pat. No. 7,655,718 B2 do not contain acid functional groups.
U.S. Pat. No. 5,912,298 (Yuho Chemicals Inc.) and U.S. Pat. No. 5,319,018 (Rohm and Hass) report floor-coating compositions comprising acid-containing polymers and metal cation crosslinkers for use as a one-coat coating system in which the polymer and the crosslinkers are mixed prior to application to the substrate.
One-coat systems that include both reactive polymers that include acid functional groups and metal cation crosslinkers need to be relatively slow reacting otherwise gelation of the coating will occur prior to application. It is often impractical to use slow reacting compositions as barrier layers in packaging applied by printing and coating applications where rapid curing is desirable. Furthermore, once mixed, the coating compositions have a limited pot life before gelation occurs. Thus it is necessary for use coating compositions to be supplied as a two-pack system wherein the reactive polymers that include acid functional groups and the metal cation crosslinkers are kept separate until needed at which point a batch of coating composition is prepared. There is often a significant amount of wastage associated with two-pack systems as it is necessary to prepare an excess coating composition the remainder of which cannot be stored for later use. WO 2010/129028 A1 (Inmat) discloses a one-coat system whereby the coating comprises (a) a dispersed, anionically functionalized matrix resin as a first aqueous dispersion; (b) a second aqueous dispersion comprising a dispersed platy mineral filler, optionally containing one or more additives; (c) a multivalent metal cation crosslinking agent added to at least one of said first or second aqueous dispersions and; (d) admixing the first and second aqueous dispersions to form the one-coat composite system. WO 2005/093000 A1 (PPG) discloses a barrier coating comprising a water-borne dispersion of a polyurethane comprising at least 30 wt % of a meta-substituted aromatic material. The barrier coating compositions may comprise crosslinkers that render the coatings thermosetting. Suitable crosslinkers are reported to include carbodiimides, aminoplasts, aziridines, zinc/zirconium ammonium carbonates and isocyanates. Good barrier properties on the coated film are imparted by the use of inorganic fillers e.g. vermiculite, mica, clays, such as microlite 923 and 963.
EP 2 172 500 A1 (Mitsubishi) discloses a one-coat polyurethane resin composition that includes an active hydrogen compound having hydroxyl groups. The resin composition may include a transition metal cation to promote an oxidation reaction of methylene groups to attain an oxygen-absorbing function.
The treatment of gas barrier coatings that include polymers comprising acid groups with a composition including multivalent metal ions has previously been employed to enhance the properties of the barrier coating. However, a platy mineral filler has been required to achieve good barrier performance. For example, WO 2010/129032 A1 (Inmat) discloses a gas barrier film comprising a matrix resin and a platy filler. The formed film is then surface treated with a multivalent metal cation crosslinking agent to stabilize the barrier film against the effects of humidity, which can potentially reduce barrier performance. The matrix resin is a water-emulsifiable polymer that includes salts of acid groups. Whilst the disclosure of WO 2010/129032 A1 is primarily directed to the use of sulfopolyesters, a wide range of possible polymers are mentioned, including polyurethanes. US 2010/0136350 A1 (Kureha) discloses a gas barrier multilayer structure, having layer (A) including a polycarboxylic acid polymer and a silicon-containing compound, derived from a silane coupling agent; and layer (B) including multivalent metal compounds e.g. a zinc compound. A two coat system in which a polyvalent metal cation is coated over a polycarboxylic acid polymer is reported to provide a gas barrier. However, the gas barrier is only sufficiently achieved in the presence of a siliane coupling agent such as tetramethoxysilane or aminopropyltrimethoxysilane, which is added to the polycarboxylic acid polymer as part of the first coat. The absence of a silane coupling agent results in a decrease of gas barrier even in the presence of a polyvalent metal cation as a second coat.